In digital cameras and the like it is important to have an optimal positional relation between a lens and an image sensor. This would include having optimal back focal distance, the distance from the rear-most lens of the lens system to the sensor image plane. An error in this back focal distance with as little as 0.01 mm would give an image with poor focus.
Another important factor is the angle of the image sensor in relation to the optical axis of the lens system. The image sensor is typically mounted within a sensor holder. This holder can be of plastic or may be, as is common, a circuit board on which the image sensor is mounted. The light sensitive plane of the image sensor should ideally be perpendicular to the optical axis of the lens system in order to provide optimal quality image. This is however difficult to achieve for instance due to set mechanical tolerances within the different parts of the camera system.
FIG. 8 shows an image sensor 810 mounted within a holder 808 with a mounting substance 816. The image sensor is not perpendicular to the optical axis as indicated by the angle A between the normal direction N of the light sensitive plane of the image sensor and the optical axis OA.